A great deal of research is currently underway to develop treatments and cures for viral infections in humans and in animals, especially for Herpes Simplex Virus (HSV) Types 1 and 2 and AIDS and AIDS related complex (ARC). Notably the incidence of AIDS and ARC in humans is increasing at an alarming rate. The five year survival rate for those with AIDS is dispiriting and AIDS patients, whose immune systems have been seriously impaired by the infection, suffer from numerous opportunistic infections including Kaposi's sarcoma and Pneumocystis carninii pnemonia. No cure for AIDS is known and current treatments are largely without adequate proof of efficacy and have numerous untoward side effects. Fear of the disease has resulted in social ostracism of and discrimination against those having or suspected of having the disease.
Retroviruses are a class of ribonucleic acid (RNA) viruses that replicate by using reverse transcriptase to form a strand of complementary DNA (cDNA) from which a double stranded, proviral DNA is produced. This proviral DNA is then randomly incorporated into the chromosomal DNA of the host cell making possible viral replication by later translation of viral message from the integrated viral genome.
Many of the known retroviruses are oncogenic or tumor causing. Indeed, the first two human retroviruses discovered, denoted human T-cell leukemia viruses I and II or HTLV-I and II, were found to cause rare leukemias in humans after infection of T-lymphocytes. The third such human virus to be discovered, HTLV-III, now referred to as HIV, was found to cause cell death after infection of T-lymphocytes and has been identified as the causative agent of AIDS and ARC.
The envelope protein of HIV is a 160 kDa glycoprotein. The protein is cleaved by a protease to give a 120 kDa external protein, gp120, and a transmembrane glycoprotein, gp41. The gp120 protein contains the amino acid sequence that recognizes the CD4 antigen on human T-helper (T4) cells.
One approach being explored is to prevent the binding of HIV to its target, the T4 cells in humans. These T4 cells have a specific region, a CD4 antigen, which interacts with gp120. If this interaction can be disrupted, the host cell infection can be inhibited.
Interference with the formation of the viral envelope glyoprotein could prevent the initial virus-host cell interaction or subsequent fusion or could prevent viral duplication by preventing the construction of the proper glycoprotein required for the completion of the viral membrane. It has been reported [See H. A. Blough et al., Biochem. Biophys. Res. Comm. 141(1), 33-38 (1986)] that the nonspecific glycosylation inhibitors 2-deoxy-D-glucose and .beta.-hydroxy-norvaline inhibit expression of HIV glycoproteins and block the formation of syncytia. Viral multiplication of HIV-infected cells treated with these agents is stopped, presumably because of the unavailability of glycoprotein required for the viral membrane formation. In another report [W. McDowell et al., Biochemistry 24(27), 8145-52 (1985)], the glycosylation inhibitor 2-deoxy-2-fluoro-D-mannose was found to inhibit antiviral activity against influenza infected cells by preventing the glycosylation of viral membrane protein. This report also studied the antiviral activity of 2-deoxyglucose and 2-deoxy-2-fluoroglucose and found that each inhibited viral protein glycosylation by a different mechanism. However, other known glycosylation inhibitors have been shown to have no antiviral activity. Thus the antiviral activity against viruses in general, and the viral activity specifically, of glycosylation inhibitors is quite unpredictable.
Research worldwide is currently underway to develop treatments and cures for HSV Types 1 and 2. Both HSV Types 1 and 2 show a predilection for infection of the ectodermal tissues wherein such infections by the virus cause lesions in the skin, oral cavity, vagina, conjunctiva, and the nervous system. Generally, infection by HSV Type 1 (HSV1) is associated with oral, facial and ocular lesions. Infection by HSV Type 2 (HSV2) generally results in genital and anal lesions. HSV infections left untreated often lead to blindness, neonatal deaths, and encephalitis. HSV Type 2 infections are at an epidemic levels in the US from venereal transmission. Greater than some twenty million persons are presently afflicted with the disease in this country with new cases and recurrences exceeding half a million annually. The annual cost of HSV infections results in a substantial economic loss to diagnose and treat. Epidemiological control of HSV is poor because the majority of the population, up to 90%, has been exposed to the virus.
Man serves as the natural host for HSV Types 1 and 2 infections whereby the virus is transmitted during close personal contact. Initial or primary infections by HSV Types 1 and 2 are contracted through breaks in the mucus membrane. In the healthy carrier the virus can be isolated in the tears, saliva, vaginal and other secretions, even during the absence of overt disease. From the mucus membrane they are able to replicate and spread to the regional lymph nodes. Occasionally these viruses can infect cells of the haemopoietic system and cause viremia.
Part of the difficulty in treating HSV infections results from the ability of these viruses to persist in a latent, or quiescent form. When the primary infection subsides or recedes, the virus generally resides in a latent form in the sensory nerve ganglia which innervate the site of primary infection. In ocular or oral infections with HSV Type 1, the virus generally resides in the trigeminal ganglia. In HSV Type 2 the virus generally resides in the sacral ganglia serving the genitalia and lower abdomen. The determinative period of latency of the HSV virus is unknown, other than this period can be upset by heat, cold, sunlight, hormonal and emotional disturbances, or by immunosuppressive agents, resulting generally in a recurrent infection.
Treatment of HSV infections have largely been ineffective. A number of strategies to stop the virus have been developed. These agents generally inhibit any one of a number of specific vital functions such as (1) adsorption, (2) uncoating, (3) transcription, (4) protein synthesis, (5) nucleic acid replication, (6) maturation, and (7) release.
Most of the antiviral agents thus far used to treat HSV infections have been compounds that interfere with viral DNA. These compounds include Idoxuridine, Cytosine Arabinoside, Adenine Arabinoside, and Trifluorothymidine. Such agents also interfere with similar host functions which results in general problems with cell toxicity and systemic use in humans. Presently, acyclovir is the preferred medication to treat infections with HSV1 and HSV2 due to its potent antiviral effect and negligable toxicity. Poor solubility at high dosage and the emergence of drug-resistant viruses, however, limit the use of this drug.
A number of RNA and DNA containing viruses have envelopes into which virus-coded glycopeptides are incorporated. HSV and cytomegalovirus (CMV) are two such enveloped viruses. Infection of a host cell by enveloped viruses initially relies on the interaction of various receptors on the host cell surface with the envelope glycoproteins of the viral membrane. Subsequently the virus and cell membranes fuse and the virion contents are released into the host cell cytoplasm. The glycoprotein containing envelope of the virus plays an important role in both the initial interaction of the virion and the host cell and in the later fusion of the viral and host cell membranes. The viral envelope seems to be derived from the cellular membrane, but the specificity is due to the viral encoded glycopeptides. Therefore, an inhibitor capable of interfering with the formation of the virus-specific membranes may prevent formation of infectious progeny virus.
It has been disclosed in U.S. application Ser. No. 295,856, filed Jan. 11, 1989, that a purified form of heparin, a sulfated polysaccharide, binds through interactions to a viral protein which is responsible for cell recognition and provides limited inhibition of host cell infection. However, heparin causes some side effects, notably hemorrhage and increased clot formation time as well as thrombocytopenia. Use of heparin is contraindicated in patients who are actively bleeding, or have hemophilia, purpura, thrombocytopenia, intracranial hemorrhage, bacterial endocarditis, active tuberculosis, increased capillary permeability, ulcerative lesions of the gastrointestinal tract, severe hypertension, threatened abortion or visceral carcinoma. The contraindication for use by hemophiliacs is particularly of concern because many such individuals are now HIV positive.
It has long been recognized that certain synthetic, water-soluble polymers exhibit a broad spectrum of biological activity [R. M. Ottenbrite in "Biological Activities of Polymers", Amer. Chem. Soc. Symp. Ser. No. 182, pp. 205-220, eds. C. E. Carraher and C. G. Gebelein (1982)]. A copolymer of divinyl ether and maleic anhydride has been shown to be active against a number of viruses and its use in cancer chemotherapy has been studied for years [Breslow, D. S. Pure and Applied Chem. 46, 103 (1976)]. Polyacrylic, polymethacrylic and a variety of other aliphatic backbone water soluble polymers also have been shown to have a broad spectrum of biological activities [W. Regelson et al., Nature 186, 778 (1960)]. Unfortunately, the extreme toxicity of these polymers has prevented their clinical use. Also, these polymers have a high molecular weight and are unable to pass through the renal membranes.
Attempts have been made to circumvent the toxicity and excretion problems by synthesis of low molecular weight (1,000 to 10,000) aliphatic polymers [R. M. Ottenbrite in "Biological Activities of Polymers", Amer. Chem. Soc. Symp. Ser. No. 182, pp. 205-220, eds. C. E. Carraher and C. G. Gebelein (1982)]. It has been found that such polymers are less toxic but have much reduced antiviral activity. These low molecular weight aliphatic polymers may be classed as "random coil" polymers. Such polymers have an unpredictable configuration because of the flexibility of the backbone linking groups. The configuration of random coil polymers in solution may be generally described as globular. Although the mechanism of action of such water-soluble polymers is unknown, one postulate is that the polymer binds to the viral membrane, e.g. the virus causing encephelomyocarditis, through an ionic attraction, thus rendering the virus unable to infect host cells.
An additional synthetic polymer approach is to place ionic groups on the backbone of a polymer which exhibits a more defined geometry. There are numerous examples of non-ionic, synthetic polymers which exhibit a more linear geometry in non-aqueous solution than do the aliphatic polymers described above [J. Macromol. Sci--Reviews in Macromol. Chem. Phys. C26(4), 551 (1986)]. The factors involved which cause this non-random coil structure are complex and poorly understood. In general, such polymers have either a very limited number of rotatable bonds which are not parallel to the polymer axis, or there is hydrogen bonding or dipolar interactions which favor linear structures. These polymers are referred to as having a "rigid backbone". A polyamide derived from terephthalic acid and p-diaminobenzene (known commercially as Kevlar.TM. supplied by DuPont) is a well-known example of such polymers.
Synthetic, water-soluble, rigid polymers are much less common, but a few high molecular weight examples are known (e.g. see U.S. Pat. Nos. 4,824,916 and 4,895,660). The non-random coil structure of this class of polymer results in high solution viscosities for a given molecular weight and concentration.
Certain anionic oligomers which inhibit viral replication without the side effects shown by heparin and known polymers have now been found. The oligomers have an ordered anion spacing, have a rigid backbone and are water-soluble. The oligomers, as polydispersed mixtures, have been described in our copending U.S. patent application Ser. No. 710,370, filed Jun. 10, 1991, and PCT patent application Ser. No. PCT/US91/04804, filed Jul. 8, 1991, the disclosures of which are hereby incorporated by reference. Although various anionic oligomers are disclosed in these applications, all the anionic oligomers were mixtures of molecular weights.
Clearly, it would be desirable to find a treatment and cure for AIDS, ARC and HSV which would display minimal or no side effects and constitute a clear improvement over the polymers previously employed as a pharmaceutical. Also such oligomers should preferably have a narrow molecular weight range, low toxicity and be easily characterized.